1. Field of the Invention
The present invention relates to a semiconductor laser, and more particularly to a semiconductor laser used as a light source for a data processing device.
2. Description of the Related Art
A semiconductor laser which is extremely compact and highly adapted to high volume production is widely used as light sources for various advanced optoelectronic devices in the fields of such as data or information processing and optical communication. Among the information processing means, the compact disks (CD) and optomagnetic disks are particularly important as means for storing and reproducing digital information as music or other data. The memory capacity in such information processing means increases as the wavelength of a light source becomes shorter. Thus, a semiconductor laser with a shorter wavelength is being actively researched. The shortest wavelength in a recent semiconductor laser which has practical characteristics is in the order of 830 nm as disclosed by Kobayashi et al in the Japanese Journal of Applied Physics, Vol. 29, No 9, September, 1990, pp. L1669-L1671.
A semiconductor laser light source with even a shorter wavelength than the above is being researched, but no sufficient practical characteristics have been obtained as yet. A semiconductor laser in which II-VI mixed crystal semiconductor is used operates only under a very low temperature in the order of 200.degree. C. below zero. A semiconductor laser light source in which a semiconductor laser, a solid laser crystal and a second harmonic generation (SHG) crystal are combined produces light of a green to a blue color, but such laser light source is more voluminous and less adapted to high volume production as compared with a discrete type semiconductor lasers.
The second harmonic generation (SHG) is based on a principle of generating a laser beam having half the wavelength of the original laser beam so that, for example, the generation of a semiconductor laser SHG having an oscillation wavelength shorter than about 1.2 .mu.m enables to produce a beam having a wavelength shorter than about 600 nm. There are reports wherein, without using SHG crystals, some attempts have been made to take out the SHG of a shorter wavelength from a discrete semiconductor laser. Disclosed in Japanese Patent Application Kokai Publication No. Sho 63(1988)-280484 is a semiconductor device in which an SHG crystal layer comprised of ZnSSe is provided on a crystal surface of a surface emitting type AlGaAs laser which produces an oscillation beam of 0.82 .mu.m whereby an SHG beam of 0.41 .mu.m is produced. Also, disclosed in the Journal of Optical Communication, Volume 35, No. 3, pp. 413-416 (1980) is an example wherein a 0.65 .mu.m red SHG beam of 0.15 nW has been produced in a Fabry-Perot type InGaAsP laser which produces an oscillation beam of 1.3 .mu.m.
In the above semiconductor device disclosed in Japanese Patent Application Kokai Publication No. Sho 63(1988)-280484, a ZnSSe layer for SHG is provided on an n-type AlGaAs contact layer of a buried type surface emitting laser structure having a double heterostructure in which a GaAs active layer is sandwiched between an n-type AlGaAs cladding layer and a p-type AlGaAs cladding layer. Since this semiconductor device employs a surface emitting laser structure, there are difficulties in lengthening the optical path length long and in obtaining a strong SHG beam.
Also, the above example disclosed in the Journal of Optical Communication relates to the observation of an SHG beam in a PCW (Plano-Convex Waveguide) laser in which, in a laminated structure, an n-type In.sub.0.9 GA.sub.0.1 AS.sub.0.22 P.sub.0.78 plano-convex waveguide layer is sandwiched between an In.sub.0.7 Ga.sub.0.4 As.sub.0.55 P.sub.0.45 active layer and an n-type InP substrate and in which a p-type InP cladding layer is further provided on the active layer. The PCW laser is a facet emitting type so that the optical path length is longer as compared with that of a surface emitting type. However, a problem therein is that, since the SHG light is extensively absorbed by the cladding layers, it is not possible to obtain a laser beam having a strong power.